Structured light systems project known light patterns onto an object. Surface contours of the object make the patterns appear distorted when viewed with a camera at a vantage point separated from the pattern projector by a baseline distance. Geometrical relationships are used to interpret the distortions to determine the distance from the projector to points on the object. In this way, three dimensional spatial coordinates of the surface of the object may be obtained.
Structured light patterns are often produced by projectors that use Texas Instruments “Digital Micromirror Devices” (DMD) as spatial light modulators. These projectors are said to use “Digital Light Processor” (DLP) technology. Examples of other kinds of micromirror arrays include those made by Reflectivity, Inc., a company that was acquired by Texas Instruments in 2006.
Micromirrors in an array are used to switch pixels in an image on or off. At any instant in time a particular pixel is either fully bright or fully dark. Pixels can be switched between the two states at rates as fast as approximately 10 kHz. DLP projectors achieve the effect of grayscale, or intermediate brightness, by pulse width modulation. Brighter pixels are the result of longer bright operation while darker pixels are the result of shorter bright operation. During a given frame of video information, a gray pixel may be produced by setting a micromirror bright for part of the frame time and dark for the remaining time. Grayscale video frames at rates of approximately 200 Hz may be obtained using pulse width modulation of bright micromirror states.
FIG. 1A is a conceptual diagram of part of a micromirror array producing grayscale via pulse width modulation. Micromirror array 105 is depicted as a 30 by 30 array; however, actual micromirror arrays may be as large as approximately 1000 by 2000 and most micromirror arrays contain at least 50,000 mirrors. FIG. 1A shows different brightness levels from bright to gray to dark at different mirrors. This appearance is obtained on an average basis over one video frame. During the frame time, brighter pixels are obtained by leaving the corresponding mirror in its bright state for longer times than darker pixels. Pulse width modulation of the bright state time determines gray levels during a video frame.
FIG. 1B is a conceptual diagram of a micromirror. Mirror 110 is supported over substrate 115 by post 120. When the mirror is position as at 110 it is said to be in its bright state. The mirror can pivot to a dark state position indicated by dashed line 125. The reflecting surface of a typical mirror measures approximately 10 μm by 10 μm and the angle between the bright and dark states states (i.e. positions 110 and 125) is about 10 degrees. (“Micromirror” is defined as a mirror smaller than 100 μm by 100 μm.) There is no intermediate mirror position that can be maintained between the two stable mirror positions that are illustrated. For this reason, spatial light modulators based on micromirrors are binary digital devices.
In structured light applications it is helpful to project spatial patterns at a rate that is high enough to enable detection techniques that reduce the effects of noise from sources such as the 60 Hz flicker of room lights. The approximately 200 Hz frame rate achievable with DLP projectors does not offer much margin above 60 Hz and other noise sources. Therefore, what are needed are systems and techniques for producing grayscale patterns with binary spatial light modulators at high rates.